There is concerted interest within the climate modeling community to define carbon emission pathways consistent with either 1.5?�C warming (goal of the Paris Climate Agreement) or 2?�C warming (upper limit of the Paris Agreement). Many Earth System Models (ESMs) and some prior data analyses suggest the airborne fraction of CO2 (AF) defined as the annual rise in global mean CO2 (CO2GM) divided by the sum of total anthropogenic fossil fuel, cement, and land use change emissions, has been rising over time. If so, this means feedbacks between the global carbon cycle and climate change are resulting in atmospheric residence of a greater fraction of human emission of CO2. Under this scenario, the limit on the cumulative human emissions of CO2 needed to limit warming to either 1.5?�C or 2?�C (relative to pre-industrial) would need to be revised downwards, relative to a model that neglects feedbacks between the global carbon cycle and climate change. We propose two types of analyses to quantify the temporal trend in AF. In year 1, we will use a multiple linear regression of CO2 growth rate as a function of anthropogenic emissions, an ENSO index, and stratospheric optical depth (SOD) to account for the effect of ENSO and volcanoes on the growth of CO2, then quantify the trend in AF based upon a time series of CO2 that has been adjusted for these two natural influences. In year 2, we propose to conduct an analysis of time series of CO2GM and the O2/N2 ratio to define changes in ocean and land uptake of atmospheric CO2. The statistical significance of the various trends will be assessed via a Monte- Carlo analysis that accounts for the uncertainties of all of the terms that enter into the analyses. The rationale of this proposed effort is that some of the prior, highly-cited studies that have quantified the trend in AF fail to adjust the data record for the influence of ENSO or volcanoes. Other studies adjust the record, albeit in a simplistic fashion, but do not provide a robust statistical analysis of the resulting trends. We have read the existing literature with great care and propose below an effort that applies a novel approach for quantitatively accounting for the effect of ENSO and volcanoes on the growth rate of atmospheric CO2, together with a state-ofthe- art statistical approach for assessing significance of the resulting trend in AF. We also propose to use measurements of the O2/N2 ratio to quantify the temporal evolution of the land and ocean sink. Our study is designed to provide clarity on how the airborne fraction of CO2 is changing, a critically important constraint for ESMs being used to guide global warming policy decisions. 1.3 Competition and Relevance AC4 aims to provide understanding of processes that govern the atmospheric abundance of trace gases. The FY2019 call states �??The most relevant proposals will be those making most use of network data and demonstrating the intrinsic value of long-term monitoring.�?� Our proposal makes use of NOAA records of CO2 (global, Mauna Loa, and South Pole), the annual growth rate for CO2, indices for ENSO, as well as the rise in ocean heat content, combined with record for stratospheric optical depth that relies on NASA satellite observations, estimates of fossil fuel, cement, and land use emission of CO2 records that NOAA has helped provide, and the O2/N2 record from many NOAA stations that is provided by colleagues at Scripps Institution of Oceanography. This proposal seems ideally aligned to the FY2019 AC4 competition.